chore(tests): unpin pytest (#2702)

chore(tests): unpin pytest

As the breaking change (deprecation) in pytest=8.1.1 has been already fixed in flexmock¹, unpin pytest and also install flexmock from the PyPI rather than RPM (to ensure the same behavior even if we switched back to the CentOS images).
Newer version of pytest introduces ‹PYTEST_VERSION› environment variable that can be used to detect whether the tests are being run or not, which is used in some safe-guards in the events-refactor².
¹ flexmock/flexmock#153
² #2590

Reviewed-by: Nikola Forró
Reviewed-by: Laura Barcziová

files/recipe-tests.yaml

@@ -8,15 +8,15 @@
     - name: Install test RPM dependencies
       ansible.builtin.dnf:
         name:
-          - python3-flexmock
           - tar
           - rsync
         state: present
     - name: Install pip deps
       ansible.builtin.pip:
         name:
           - requre
-          - pytest==8.0.2
+          - flexmock
+          - pytest
           - pytest-cov
           - pytest-flask
           - deepdiff < 8.0.0 # version 8.0.0 requires numpy, avoid it